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drm: Push latency sensitive bits of vblank scanoutpos timestamping into kms drivers. 

A change in locking of some kms drivers (currently intel-kms) make
the old approach too inaccurate and also incompatible with the
PREEMPT_RT realtime kernel patchset.

The driver->get_scanout_position() method of intel-kms now needs
to aquire a spinlock, which clashes badly with the former
preempt_disable() calls in the drm, and it also introduces larger
delays and timing uncertainty on a contended lock than acceptable.

This patch changes the prototype of driver->get_scanout_position()
to require/allow kms drivers to perform the ktime_get() system time
queries which go along with actual scanout position readout in a way
that provides maximum precision and to return those timestamps to
the drm. kms drivers implementations of get_scanout_position() are
asked to implement timestamping and scanoutpos readout in a way
that is as precise as possible and compatible with preempt_disable()
on a PREMPT_RT kernel. A driver should follow this pattern in
get_scanout_position() for precision and compatibility:

spin_lock...(...);
preempt_disable_rt(); // On a PREEMPT_RT kernel, otherwise omit.
if (stime) *stime = ktime_get();
... Minimum amount of MMIO register reads to get scanout position ...
... no taking of locks allowed here! ...
if (etime) *etime = ktime_get();
preempt_enable_rt(); // On PREEMPT_RT kernel, otherwise omit.
spin_unlock...(...);

v2: Fix formatting of new multi-line code comments.

Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
This commit is contained in:
Mario Kleiner 2013-10-30 05:13:06 +01:00 committed by Dave Airlie
parent 862cc8b4bb
commit 8f6fce03dd
2 changed files with 20 additions and 10 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

20
drivers/gpu/drm/drm_irq.c
View File

@ -219,7 +219,7 @@ int drm_vblank_init(struct drm_device *dev, int num_crtcs)
for (i = 0; i < num_crtcs; i++)
init_waitqueue_head(&dev->vblank[i].queue);
DRM_INFO("Supports vblank timestamp caching Rev 1 (10.10.2010).\n");
DRM_INFO("Supports vblank timestamp caching Rev 2 (21.10.2013).\n");
/* Driver specific high-precision vblank timestamping supported? */
if (dev->driver->get_vblank_timestamp)
@ -586,14 +586,17 @@ int drm_calc_vbltimestamp_from_scanoutpos(struct drm_device *dev, int crtc,
* code gets preempted or delayed for some reason.
*/
for (i = 0; i < DRM_TIMESTAMP_MAXRETRIES; i++) {
/* Get system timestamp before query. */
stime = ktime_get();
/*
* Get vertical and horizontal scanout position vpos, hpos,
* and bounding timestamps stime, etime, pre/post query.
*/
vbl_status = dev->driver->get_scanout_position(dev, crtc, &vpos,
&hpos, &stime, &etime);
/* Get vertical and horizontal scanout pos. vpos, hpos. */
vbl_status = dev->driver->get_scanout_position(dev, crtc, &vpos, &hpos);
/* Get system timestamp after query. */
etime = ktime_get();
/*
* Get correction for CLOCK_MONOTONIC -> CLOCK_REALTIME if
* CLOCK_REALTIME is requested.
*/
if (!drm_timestamp_monotonic)
mono_time_offset = ktime_get_monotonic_offset();
@ -604,6 +607,7 @@ int drm_calc_vbltimestamp_from_scanoutpos(struct drm_device *dev, int crtc,
return -EIO;
}
/* Compute uncertainty in timestamp of scanout position query. */
duration_ns = ktime_to_ns(etime) - ktime_to_ns(stime);
/* Accept result with < max_error nsecs timing uncertainty. */

10
include/drm/drmP.h
View File

@ -836,12 +836,17 @@ struct drm_driver {
/**
* Called by vblank timestamping code.
*
* Return the current display scanout position from a crtc.
* Return the current display scanout position from a crtc, and an
* optional accurate ktime_get timestamp of when position was measured.
*
* \param dev DRM device.
* \param crtc Id of the crtc to query.
* \param *vpos Target location for current vertical scanout position.
* \param *hpos Target location for current horizontal scanout position.
* \param *stime Target location for timestamp taken immediately before
* scanout position query. Can be NULL to skip timestamp.
* \param *etime Target location for timestamp taken immediately after
* scanout position query. Can be NULL to skip timestamp.
*
* Returns vpos as a positive number while in active scanout area.
* Returns vpos as a negative number inside vblank, counting the number
@ -858,7 +863,8 @@ struct drm_driver {
*
*/
int (*get_scanout_position) (struct drm_device *dev, int crtc,
int *vpos, int *hpos);
int *vpos, int *hpos, ktime_t *stime,
ktime_t *etime);
/**
* Called by \c drm_get_last_vbltimestamp. Should return a precise